1. Field of the Invention
The present invention relates to a signal processing circuit and a physical quantity measuring device using the signal processing circuit, and particularly relates to a magnetic element control device that drives a time-resolution flux-gate type (hereinafter, referred to as an FG-type) magnetic element, a magnetic element control method, and a magnetic detection device that detects a magnetic field using the magnetic element control method.
2. Description of the Related Art
Generally, FG-type magnetic elements have a high sensitivity of detecting a magnetic field and are capable of a reduction in size, as compared to Hall elements or magneto-resistive elements which are magnetic elements that detect similar magnetism, and thus are used in azimuth detection devices such as portable electronic devices, and the like.
FIG. 12 is a diagram showing a configuration example of a time-resolution FG-type magnetic element (for magnetic balance type measurement). As shown in FIG. 12, the FG-type magnetic element in the magnetic balance type measurement is configured such that a feedback (hereinafter, referred to as FB) winding coil is wound around the outer circumferential surface of a magnetic substance core which is formed of a high magnetic permeability material, in addition to an excitation winding and a detection winding. A region around which the excitation winding is wound is driven by an excitation signal as an exciting coil, a region around which the detection winding is wound outputs a detection signal as a detection coil, and a region around which a feedback winding is wound is driven by a feedback signal as a feedback coil.
FIG. 13 is a waveform diagram showing a principle of a magnetic balance type measurement in which magnetism is detected using the time-resolution FG-type magnetic element.
PART (a) of FIG. 13 shows an excitation current which is supplied to the exciting coil of the magnetic element, in which the vertical axis thereof represents a current value of the excitation current, and the horizontal axis thereof represents time. The excitation current is a positive and negative alternating signal bordered by a reference current value of 0 A (zero amperes). PART (b) of FIG. 13 shows an FB signal (that is, a feedback signal) which is a current applied to the FB coil of the magnetic element, in which the vertical axis thereof represents a current value of the FB signal, and the horizontal axis thereof represents time. PART (c) of FIG. 13 shows the voltage value of a pulse (hereinafter, also called a pickup signal pu) which is generated by the detection coil of the magnetic element due to an induced electromotive force, in which the horizontal axis thereof represents time.
As shown in FIG. 13, in the case of magnetic balance type measurement, a magnetic field that cancels out the stationary magnetic field (stationary magnetic field passing through the magnetic substance core) which is applied to the magnetic element is generated by the above FB coil
A stationary magnetic field which is applied to the magnetic element is measured from a current value when the magnetic field that cancels out a stationary magnetic field is generated in the FB coil.
In a magnetic balance system, as a coil that generates a magnetic field that cancels out a stationary magnetic field in the magnetic substance core, the above FB coil is provided in the magnetic element, in addition to the exciting coil and the detection coil.
Hereinafter, in this specification, a method in which a stationary magnetic field in the magnetic substance core is canceled by applying an FB signal and in which a magnetic field is measured is referred to as FB control of an FB coil.
In addition, in the case of the magnetic balance type measurement, a time interval between pulses generated by the detection coil is measured in the positive and negative alternating time zone of the excitation signal which is applied to the exciting coil. The FB signal is applied to the FB coil so that time from time t1 at which the measured detection signal of a negative voltage is output to time t2 at which the detection signal of a positive voltage is detected becomes equal to T/2.
For example, in PART (c) of FIG. 13, when a time width between time t1 and time t2 is larger than T/2, the stationary magnetic field in a negative direction is applied as shown in PART (a) of FIG. 13, and the curve of the excitation signal changes substantially from curve L0 to curve L2. For this reason, since curve L2 of the excitation signal is returned to a position of curve L0 in which the time width between time t1 and time t2 becomes equal to T/2, the FB signal of the current value of line FB2 in PART (b) of FIG. 13 is applied to the FB coil.
On the other hand, in PART (c) of FIG. 13, when the time width between time t1 and time t2 is smaller than T/2, the stationary magnetic field in a positive direction is applied as shown in PART (a) of FIG. 13, and the curve of the excitation signal changes substantially from curve L0 to curve L1. For this reason, since curve L1 of the excitation signal is returned to the position of curve L0, the FB signal of the current value of line FB1 in PART (b) of FIG. 13 is applied to the FB coil.
The intensity of the stationary magnetic field which is applied to the magnetic element is obtained from the current value of the FB signal applied to the FB coil so that the time width between time t1 and time t2 becomes equal to T/2.
Meanwhile, the above-mentioned description has been given of a case where the vertical axis component in PART (a) of FIG. 13 is set to a current and the excitation signal which is applied to the exciting coil is a current signal, but the vertical axis component may be represented as a voltage value between both ends of the terminal of the exciting coil. In this case, in the PART (a) of FIG. 13, the voltage of the vertical axis intersecting the horizontal axis is set to a reference voltage and is represented by Vref (which is 0 A in current notation).
Next, FIG. 14 is a block diagram showing an example configuration of a magnetic detection device using a magnetic element control device in FB coil FB control. In FIG. 14, a magnetic element 100 is constituted by a detection coil 1001, an exciting coil 1002, and an FB coil 1003.
A magnetic element control device 200 is constituted by a magnetic element control unit 201, a clock signal generation unit 202, and a clock signal adjustment unit 203.
The clock signal generation unit 202 generates a clock of cycle T, and outputs the generated clock to the clock signal adjustment unit 203.
The clock signal adjustment unit 203 adjusts the signal level of the clock to be supplied, and outputs the adjusted clock to the magnetic element control unit 201.
The magnetic element control unit 201 includes a detection signal amplification unit 2011, a detection signal comparison unit 2012, a feedback signal adjustment unit 2013, a feedback signal conversion unit 2014, a data signal conversion unit 2015, an excitation signal adjustment unit 2016, and an excitation signal generation unit 2017.
The excitation signal generation unit 2017 generates a triangular wave as the excitation signal shown in PART (a) of FIG. 13 from a clock which is supplied from the clock signal adjustment unit 203.
The excitation signal adjustment unit 2016 adjusts the voltage level of the excitation signal which is supplied from the excitation signal generation unit 2017, and supplies the adjusted voltage level, as the excitation signal, to the exciting coil.
The exciting coil 1002 generates a magnetic field corresponding to the triangular wave within the magnetic substance core of the magnetic element 100.
The detection coil 1001 generates a pulse at the positive and negative alternating time zone of the excitation signal in the magnetic substance core.
The detection signal amplification unit 2011 amplifies the voltage level of the pulse which is supplied from the detection coil, and outputs the amplified voltage level, as the detection signal, to the detection signal comparison unit 2012.
The detection signal comparison unit 2012 obtains a difference between T/2 and the time width of the pulse (detection signal) between time t1 and time t2, and outputs the difference to the feedback signal conversion unit 2014.
The feedback signal conversion unit 2014 obtains the current value of the FB signal, supplied to the FB coil, from the supplied difference.
Here, the feedback signal conversion unit 2014 reads out the current value corresponding to the difference from an FB current value table which is previously written and stored in an internal storage unit, and obtains the current value of the FB signal.
The FB current value table is a table indicating the correspondence of the above difference to a current value (digital value) for cancel a stationary magnetic field in the magnetic substance core.
The feedback signal adjustment unit 2013 performs D/A (Digital/Analog) conversion on the current value of the FB signal which is supplied from the feedback signal conversion unit 2014, and outputs the generated current as the FB signal to the FB coil 1003. In addition, the feedback signal adjustment unit 2013 outputs the current value of the FB signal, supplied from the feedback signal conversion unit 2014, to the data signal conversion unit 2015.
The data signal conversion unit 2015 obtains the intensity of the stationary magnetic field canceled in the magnetic substance core, that is, the intensity of the stationary magnetic field applied to the magnetic element 100, from the current value of the FB signal to be supplied. Here, the data signal conversion unit 2015 reads out the magnetic field intensity corresponding to the current value of the FB signal, from a current value magnetic field table which is previously written and stored in an internal storage unit, and obtains the intensity of the magnetic field which is applied to the magnetic element 100. The current value magnetic field table is a table indicating the correspondence of the above current value of the FB signal to the intensity of the applied stationary magnetic field.
When a magnetic field in the magnetic balance system is detected using the time-resolution FG-type magnetic element, a magnetic field within the magnetic substance core is maintained in an equilibrium state so that the detection signal is output at a constant time interval (T/2) regardless of the stationary magnetic field which is applied to the magnetic element 100. For this reason, a restriction can be performed by the power supply voltage of the entire magnetic element 100, that is, the measurement of the magnetic field can be performed in a range in which the current value of the FB signal is capable of being supplied.
In addition, when the magnetic of the magnetic balance system is detected using the time-resolution FG-type magnetic element, the magnetic field dependency of excitation efficiency is small as the characteristics of the magnetic element, and thus the waveform of the detection signal and the stationarity of a time interval at which the detection signal is generated have a tendency to be maintained.
For this reason, when a measuring object is applied to the magnetic element that measures a magnetic field which is generated by a current of approximately several hundred A (amperes) in the entire measurement current range in a state where linearity is maintained, magnetic field detection in the magnetic balance system is mainly used (see, for example, Japanese Unexamined Patent Applications, First Publications No. 2008-292325, No. 2007-078423, and No. 2007-078422).
When the magnetic field is detected by the magnetic balance system using the time-resolution FG-type magnetic element described above, the FB signal is generally performed by current control in FB control of an FB coil.
As previously stated, even when there is a proportional relation between the current value in an FB control signal and the intensity of a magnetic field generated by the current value, and the resistance of the FB coil (hereinafter, also referred to as a feedback coil) changes corresponding to a temperature due to the difference in the current value of the FB signal, the current value of the FB signal is controlled at a constant current. For this reason, in the magnetic field having a high intensity in which the current value of the FB signal increases, it is also possible to maintain the sensitivity linearity of the magnetic element.
In addition, even when each excitation efficiency of the exciting coil and the feedback coil changes with the individual deviation of the characteristics of the magnetic element, the convergence state of magnetic field equilibrium between the magnetic field generated by the FB signal and the stationary magnetic field is restricted by the characteristics of the control circuit that outputs the FB signal, and a residual error (error) in convergence is not generated.
Further, when the ratio of the excitation efficiency of the exciting coil to the excitation efficiency of the feedback coil is held constant, the magnetic sensitivity ratio of the exciting coil to the feedback coil does not change, and thus the convergence time until the magnetic field based on the FB signal and the stationary magnetic field reach magnetic field equilibrium also does not change.
Therefore, when the exciting coil and the feedback coil in the magnetic element are simultaneously formed by a semiconductor process or the like, a coil resistance ratio is maintained even in a case where each resistance of the exciting coil and the feedback coil changes. Thus, a residual error in an equilibrium state which is an index of the convergence of magnetic field equilibrium does not occur, and the time to reach the equilibrium state does not change.
However, when a magnetic field is detected by the magnetic balance type using the time-resolution FG type magnetic element, and the FB signal controls the intensity of a magnetic field generated by the feedback coil based on a current value, the current value corresponding to the intensity of a magnetic field is required to be determined by controlling constant current. For this reason, a voltage-to-current conversion circuit that controls a constant current has to be mounted.
FIG. 15 is a diagram showing a configuration of a voltage-to-current conversion circuit. In addition, FIG. 16 is a diagram showing the voltage-to-current conversion circuit.
As shown in FIG. 15, the voltage-to-current conversion circuit includes an excitation triangular wave generation circuit 2017a constituting the above-mentioned excitation signal generation unit 2017, and the excitation signal adjustment unit 2016.
The excitation triangular wave generation circuit 2017a generates a triangular wave (having a voltage level Vex), as the excitation signal shown in FIG. 16, from a clock which is supplied from the clock signal adjustment unit 203.
The excitation signal adjustment unit 2016 includes a difference amplifier 2001 and a resistor 2002 (resistance value R).
The excitation signal adjustment unit 2016 converts the triangular wave Vex, and generates a constant current (current value Iex) flowing through the exciting coil 1002 (resistance value Rex).
In the resistor 2002, the first end thereof is connected to the output of the excitation triangular wave generation circuit 2017a, and the second end thereof is connected to the inverting input terminal of the difference amplifier 2001.
The difference amplifier 2001 is configured such that the non-inverting input terminal thereof is connected to a reference voltage source which is not shown and a reference voltage Vref is input thereto, and that the inverting input terminal thereof is connected to the second end of the resistor 2002.
The exciting coil 1002 is connected between the output terminal and the inverting input terminal of the difference amplifier 2001.
According to the above configuration, in a current-voltage conversion circuit, when the voltage level (which is set to V−) of the non-inverting input terminal and the voltage level Vex of the triangular wave change, an current Iex is caused to flow to the exciting coil 1002 so that the relation of V−=Vref is established. That is, in a range where the peak value of an exciting voltage (voltage between both ends of the exciting coil 1002) is Iex×Rex<Vcc/2 when the power supply voltage level of the difference amplifier 2001 is set to Vcc, the relation of Iex=(Vex−Vref)/R is established. Thereby, since voltage control is performed on Vex, the excitation signal for current control is generated.
Incidentally, as described above, when the triangular wave is used in the excitation signal, signal distortion (hereinafter, called crossover distortion) shown in FIG. 16 is generated by an amplifier used in the excitation triangular wave generation circuit 2017a. Hereinafter, the generation of the signal distortion will be described.
In FIG. 16, the excitation signal shows the waveform of a current flowing through the exciting coil 1002, and shows a state where the crossover distortion is generated in the excitation signal at a time of switching from positive to negative and a time of switching from negative to positive.
The crossover distortion occurs because crossover distortion generated in the triangular wave voltage Vex which is generated by the excitation triangular wave generation circuit 2017a is reflected. Consequently, there is considered a method of suppressing distortion described below through the amplifier used in the excitation triangular wave generation circuit 2017a. 
For example, when the amplifier used in the excitation triangular wave generation circuit 2017a is a class A amplifier, there is considered a method of applying a bias current not to generate the crossover distortion in the triangular wave which is an output signal, so that the crossover distortion does not occur in the vicinity of a reference current value of 0 A of the excitation current. However, in such a method, it is necessary to cause a bias current to flow steadily, and thus the power consumption of the entire device increases.
On the other hand, when the amplifier used in the excitation triangular wave generation circuit 2017a is a class B amplifier, since the amplifier is an amplifier of a type in which a bias current is not applied, crossover distortion occurs in the triangular wave which is an output signal, it is not possible to avoid the occurrence of the crossover distortion in the vicinity of a reference current value of 0 A of the excitation current.
Generally, as the amplifier used in the excitation triangular wave generation circuit 2017a, an amplifier called a class AB amplifier that performs an intermediate operation between class A and class B mentioned above is used. For this reason, when a current-voltage conversion circuit is formed using a class AB amplifier having a small drive current for the purpose of a reduction in power consumption, the crossover distortion occurs in the vicinity of a reference current value of 0 A of the excitation current.
As described above, the detection signal comparison unit 2012 obtains a time width between a time at which the pulse (pu signal) switches from positive to negative and a time at which the pulse switches from negative to positive, that is, a difference between T/2 and a time width from a first detection signal shown in FIG. 16 from a second detection signal, and outputs the resultant to the feedback signal conversion unit 2014. In addition, the feedback signal conversion unit 2014 obtains the current value of the FB signal which is supplied to the feedback coil, from the supplied difference.
In FIG. 16, when a stationary magnetic field is not applied to the magnetic element 100, the current value which is supplied to the exciting coil 1002 shows a change corresponding to curve L0c. In addition, when the time width of the pu signal is larger than T/2, a stationary magnetic field in a negative direction is applied, and the curve of the excitation signal substantially changes from curve L0c to curve L2c. In addition, when the time width of the pu signal becomes smaller than T/2, a stationary magnetic field in a positive direction is applied, and the curve of the excitation signal substantially changes from curve L0c to curve L1c.
Among these curves, curve L0c transects 0 A which is a reference current value at a time zone when the crossover distortion is generated in the excitation signal. For this reason, the pu signal is generated in a period when linearity is not present in the excitation current. That is, a period T/2 when the detection signal comparison unit 2012 is used in a difference arithmetic operation includes a region in which the crossover distortion is generated in the excitation signal, and thus is set to a value in which the detection accuracy of a stationary magnetic field is reduced.
As stated above, in the magnetic element control device 200 shown in FIG. 15, since the pu signal is generated at a time when the crossover distortion is generated, the linearity of magnetic sensitivity deteriorates, and output stability at a constant temperature and in a constant external magnetic field deteriorates.
The present invention is contrived in view of such circumstances, and an object thereof is that, in a magnetic element control device that detects a magnetic field of a magnetic balance system using a time-resolution FG type magnetic element, the output stability of the magnetic element control device is improved without being influenced by signal distortion which is generated in an excitation signal or a feedback signal.